When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias includes the use of an implantable pulse generator. An implantable pulse generator is implanted in the patient and delivers therapy to the patient's heart under certain predetermined conditions.
An implantable pulse generator can be, among other things, a pacemaker. A pacemaker delivers timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular lead (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. A pacemaker is often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
An implantable pulse generator can also be a cardioverter/defibrillator. A cardioverter/defibrillator is capable of delivering higher energy electrical stimuli to the heart. The cardioverter/defibrillator is often used to treat a patient with a tachyarrhythmia, that is, a heart that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A cardioverter/defibrillator is capable of delivering a high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to the pacemaker and the cardioverter/defibrillator, an implantable pulse generator can combine the function of the pacemaker and the cardioverter/defibrillator, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
A typical pacemaker and/or cardioverter/defibrillator include one or more electrical leads, which extend from a sealed housing of the pulse generator through the venous system into the inner walls of a heart. Within the housing are a battery for supplying power, a capacitor for delivering bursts of electric current through the leads to the heart, and monitoring circuitry for monitoring the heart and determining not only when and where to apply the current bursts but also their number and magnitude. The monitoring circuitry generally includes a microprocessor and a memory that stores instructions directing the microprocessor to interpret electrical signals that naturally occur in the heart as normal or abnormal rhythms. For abnormal rhythms, the instructions, or more generally signal-processing algorithm, tell the processor what, if any, electrical therapy should be given to restore normal heart function.
In general, these algorithms use the time intervals between successive heart beats, or cardiac events, as key determinants of therapy decisions. Thus, to ensure the validity of therapy decisions, it is very important to ensure accuracy of these intervals. Determining these intervals can be especially problematic in a dual-chamber pacemaker and/or cardioverter/defibrillator. In the dual chamber device, a cardiac signal is monitored from two chambers of the heart, such as the right ventricle and the right atrium. In this device, there is a significant risk of mistaking a ventricle beat for an atrial beat, and therefore counting too many atrial beats and miscalculating some atrial intervals (the time between atrial beats).
In addition to mistaking sensed signals from cardiac chambers, “noise” from non-cardiac sources can also be problematic for the pacemaker and/or cardioverter/defibrillator. Noise interferes with the proper operation of the device, and is most commonly caused by external electrical interference. The response of the implanted device in the presence of noise, or interference, is important in ensuring the device is ready to provide therapy to the patient regardless of the presence of noise.
Thus, there is a need in the art for effective ways of addressing noise sensed in cardiac signals sensed by an implanted pulse generator, and maintaining a proper response of the implantable pulse generator in the presence of such noise.